Mechanism for coordinate regulation of by sRNA-sRNA interaction in .

RpoS is a key regulator of general stress responses in . Its expression is post-transcriptionally up-regulated by the small RNAs (sRNAs), ArcZ, DsrA and RprA, through sRNA- mRNA interactions. Although overexpression of the sRNA, CyaR, was reported to down-regulate expression, how CyaR regulates has not been determined. Here, we report that CyaR represses expression by base-pairing with a region next to the ArcZ binding site in the 5' UTR of mRNA and that CyaR expression itself is down-regulated by ArcZ through sRNA-sRNA interaction. The short form of ArcZ, but not the full-length form, can base-pair with CyaR. This ArcZ-CyaR interaction triggers degradation of CyaR by RNase E, alleviating the CyaR-mediated repression. These results suggest that ArcZ not only participates in translation as an activator, but also acts as a regulator of the reciprocally acting CyaR, maximizing its activating effect.